This invention relates to photography. More particularly, the invention is directed to the hypersensitization of photographic film for purposes of increasing its sensitivity to electromagnetic radiation, and decreasing its low intensity reciprocity law failure. Photographic film treated in accordance with the invention disclosed herein is particularly suitable for photography of astronomical objects.
It is known that the speed of photographic emulsions may be increased by subjecting them prior to exposure to various gases under pneumatic pressure, including carbon dioxide, nitrogen, and hydrogen. Such a method is taught in an article in Journal of the Optical Society of America, Volume 34, pages 285-289 (1944), entitled "Influence of Pneumatic Pressure on the Photographic Sensitivity" by Choong Shin-Piaw. Other methods of increasing the sensitivity of photographic emulsions include treatment with various chemical sensitizers, and also the application of high vacuum to the emulsions prior to exposure. All of these methods result in a relatively small increase in sensitivity of the emulsions, and they do not eliminate the problem of reciprocity law failure at very low light intensities. This problem is particularly important in the photography of astronomical objects which are very distant and have extremely small luminosities.
It is also known that a substantial increase in the sensitivity of photographic emulsions can be attained by baking prior to exposure, both in vacuum and in various gaseous atmospheres. Nitrogen has been found to provide a satisfactory baking environment, particularly for astronomical applications. However, baking of emulsions produces the undesirable side effect of increased fog, which constitutes a serious limitation on the use of this method.
The treatment of photographic emulsions in an atmosphere of hydrogen has been shown to result in both a substantial increase in sensitivity and a marked reduction in low-intensity reciprocity law failure of such treated emulsions. Especially desirable results have been attained by evacuating the atmosphere surrounding the emulsions to a very high vacuum, thus removing the oxygen and moisture from the photosensitive layers, followed by soaking of the emulsions in a hydrogen atmosphere. Since pure hydrogen is very dangerous because of its propensity to form an explosive mixture with oxygen or air, it is desirable to employ only dilute mixtures of hydrogen with inert gases, such as nitrogen, in the hydrogen soaking treatment. Such mixtures, termed "forming gases", have been employed with hydrogen concentrations as low as 10 volume percent in soaking photographic emulsions at room temperature to achieve increased sensitivity. The treatment of photographic emulsions by this method does not require any substantial pressure of hydrogen or forming gas above atmospheric pressure.
Still lower concentrations of hydrogen have been used in the forming gas treatment of photographic emulsions by baking the emulsions during the treatment, or alternatively by subjecting the emulsions to a heated current of forming gas. Baking or gas temperatures in these processes range from 35.degree. C. to about 80.degree. C. and are preferably approximately 60.degree. C. These methods suffer from the drawback of increased fog in the emulsions, and the resulting fog densities are greater than those encountered in the nitrogen baking method. In fact, even hydrogenation of the emulsions without baking generally produces a fog density greater than that from nitrogen baking for the same period of time. Similar results have been obtained by various sequential combinations of baking in forming gas or nitrogen and hydrogenation. In the case of colored photographic emulsions, hydrogenation has been shown to be somewhat superior to baking from the standpoint of accuracy of color rendition of the hypersensitized film.
An important limitation common to all of the methods discussed above is that they are suitable only for photographic emulsions with unobstructed surfaces to provide free access to the sensitizing gas. These processes are employed with photographic plates, or with film wound on a processing reel to maintain a gap adjacent to the emulsion surface through which the sensitizing gas can flow. If the film is wound tightly in a cartridge or roll the inner portions of the emulsion will become less sensitized than the outer portions, and unsatisfactory results will be obtained from all of the above methods. In addition, treatment of such film by any of these methods will result in an unacceptably high level of film fog.